# std::ranges::cdata

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
Defined in header `<iterator>` |  |
inline namespace /*unspecified*/ { inline constexpr /*unspecified*/ cdata = /*unspecified*/; } |  | (since C++20) (customization point object)
Call signature |  |
template< class T > requires /* see below */ constexpr /* see below */ cdata( T&& t ); |  | (since C++20)

Returns a pointer to the first element of constant type(since C++23) of a contiguous range denoted by a const-qualified(until C++23) argument.

Let `CT` be const std::remove_reference_t<T>& if the argument is an lvalue (i.e. `T` is an lvalue reference type), const T otherwise. A call to `ranges::cdata` is expression-equivalent to ranges::data(static_cast<CT&&>(t)). The return type is equivalent to std::remove_reference_t<ranges::range_reference_t<CT>>*. | (until C++23)

If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to `ranges::cdata` is expression-equivalent to: `as-const-pointer`(ranges::data(`possibly-const-range`(t))). The return type is equivalent to std::remove_reference_t<ranges::range_const_reference_t<T>>*. In all other cases, a call to `ranges::cdata` is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation. | (since C++23)

If ranges::cdata(t) is valid, then it returns a pointer to an object of constant type(since C++23).

### Customization point objects

The name `ranges::cdata` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__cdata_fn`.

All instances of `__cdata_fn` are equal. The effects of invoking different instances of type `__cdata_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `ranges::cdata` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `ranges::cdata` above, `__cdata_fn` models

- std::invocable<__cdata_fn, Args...>,
- std::invocable<const __cdata_fn, Args...>,
- std::invocable<__cdata_fn&, Args...>, and
- std::invocable<const __cdata_fn&, Args...>.

Otherwise, no function call operator of `__cdata_fn` participates in overload resolution.

### Example

Run this code

    #include <cstring>
    #include <iostream>
    #include <ranges>
    #include <string>

    int main()
    {
        std::string src {"hello world!\n"};

    //  std::ranges::cdata(src)[0] = 'H'; // error, src.data() is treated as read-only
        std::ranges::data(src)[0] = 'H'; // OK, src.data() is a non-const storage

        char dst[20]; // storage for a C-style string
        std::strcpy(dst, std::ranges::cdata(src));
        // [data(src), data(src) + size(src)] is guaranteed to be an NTBS

        std::cout << dst;
    }

Output:

    Hello world!

### See also

ranges::data (C++20) | obtains a pointer to the beginning of a contiguous range (customization point object)
data (C++17) | obtains the pointer to the underlying array (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/ranges/cdata&oldid=179363"